OSI model, TCP/IP, and encapsulation

Where the models came from

ISO created the OSI model as a vendor-neutral framework. TCP/IP was already deployed and became the de facto standard, so OSI survives as the reference and teaching model.

Why layered models at all: they manage complexity, specify requirements, keep engineering modular, contain changes, encourage development, and make the whole thing easier to teach and learn.

Host-to-host basics

Source, transmission media, destination. Every message carries source and destination addresses (the envelope analogy).

OSI layers

Lower layers (1-4) move the data. Upper layers (5-7) deal with the content and its presentation.

• 7 Application, network services to apps: closest to the user. The browser uses L7 protocols, it doesn’t live there. The only layer that serves no other layer.
• 6 Presentation, data representation: a common format both ends can read. Compression and encryption (which can also happen lower).
• 5 Session, interhost communication: opens, manages, and ends sessions (dialogs). Direction, checkpoints, recovery. Explicit in RPC apps.
• 4 Transport, end-to-end connections: segmentation and reassembly, flow control, reliable or unreliable delivery. Logical ports, local to the host.
• 3 Network, data delivery: path selection from source to final destination. Logical addressing, meaning IP addresses.
• 2 Data Link, access to media: framing, media access, error detection. NIC-to-NIC on the same subnet. MAC addresses.
• 1 Physical, binary transmission: encoding, voltages, timing, rates, distances, connectors. The only hardware-only layer.

Reliable vs. unreliable at L4: a file transfer needs reliable; a video stream can drop a pixel unnoticed.

TCP/IP stack

• Application (OSI 5-7): represents data to users, encoding, dialog control.
• Transport (OSI 4): end-device to end-device communication.
• Internet (OSI 3): logical addressing, best path.
• Link (OSI 1-2): hardware and media.

Encapsulation and PDUs

Down the stack on the sender is encapsulation: each layer adds its header, and the link layer adds a trailer (FCS). Up the stack on the receiver is de-encapsulation: strip the header, hand the payload up.

Each layer talks logically to its peer layer on the other host. The L5-7 content is the payload.

The PDU names by layer:

• Application: data
• Transport: segment
• Internet / Network: packet
• Link / Data Link: frame
• Physical: bits

Quick recall

• L2 = MAC, L3 = IP, L4 = ports.
• Down = encapsulate, up = de-encapsulate.
• Data → segment → packet → frame → bits.